<!DOCTYPE html>
<html>
<head>
    <meta charset='utf-8' />
    <title>Create a hover effect</title>
    <meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
    <script src='https://api.tiles.mapbox.com/mapbox-gl-js/v0.47.0/mapbox-gl.js'></script>
    <link href='https://api.tiles.mapbox.com/mapbox-gl-js/v0.47.0/mapbox-gl.css' rel='stylesheet' />
    <style>
        body { margin:0; padding:0; }
        #map { position:absolute; top:0; bottom:0; width:100%; }
    </style>
</head>
<body>

<div id='map'></div>
<script>
mapboxgl.accessToken = 'pk.eyJ1IjoiaWRheHl0IiwiYSI6ImNqamlva3YzcjAwcXUzcXF3NXVjNmRxZTcifQ.4bA_AtQCKSy_9j-v4CfMDw';
var map = new mapboxgl.Map({
    container: 'map',
    style: 'mapbox://styles/mapbox/streets-v9',
    center: [-100.486052, 37.830348],
    zoom: 2
});
var hoveredStateId =  null;

map.on('load', function () {
    map.addSource("states", {
        "type": "geojson",
        "data": "https://www.mapbox.com/mapbox-gl-js/assets/us_states.geojson"
    });

    // The feature-state dependent fill-opacity expression will render the hover effect
    // when a feature's hover state is set to true.
    map.addLayer({
        "id": "state-fills",
        "type": "fill",
        "source": "states",
        "layout": {},
        "paint": {
            "fill-color": "#627BC1",
            "fill-opacity": ["case",
                ["boolean", ["feature-state", "hover"], false],
                1,
                0.5
            ]
        }
    });

    map.addLayer({
        "id": "state-borders",
        "type": "line",
        "source": "states",
        "layout": {},
        "paint": {
            "line-color": "#627BC1",
            "line-width": 2
        }
    });

    // When the user moves their mouse over the state-fill layer, we'll update the
    // feature state for the feature under the mouse.
    map.on("mousemove", "state-fills", function(e) {
        if (e.features.length > 0) {
            if (hoveredStateId) {
                map.setFeatureState({source: 'states', id: hoveredStateId}, { hover: false});
            }
            hoveredStateId = e.features[0].id;
            map.setFeatureState({source: 'states', id: hoveredStateId}, { hover: true});
        }
    });

    // When the mouse leaves the state-fill layer, update the feature state of the
    // previously hovered feature.
    map.on("mouseleave", "state-fills", function() {
        if (hoveredStateId) {
            map.setFeatureState({source: 'states', id: hoveredStateId}, { hover: false});
        }
        hoveredStateId =  null;
    });
});
</script>

</body>
</html>